Developing sustainable ionomers as alternatives to perfluorinated ones like Nafion is critical for advancing eco-friendly energy technologies. This study introduces lignosulfonic acid (LS) as a potential ionomer to address proton conductivity limitations in thin-film formats. Through sulfomethylation of Kraft lignin, LS ionomers with tailored ion exchange capacities (IECs: 0.6, 1.7, and 2.9 meq/g) were synthesized and systematically characterized for their structural, thermal, and electrochemical properties. Among them, LS ionomers with an IEC of 1.7 (LS 1.7) exhibited the highest proton conductivity (90 mS/cm), significantly outperforming Nafion (8 mS/cm) in 200-240 nm thick films under high humidity (80-90%) conditions. This enhanced performance was attributed to optimal ionic group distribution and well-developed ionic domains, as evidenced by transmission electron microscopy (TEM) and atomic force microscopy (AFM) analyses. Confocal laser scanning microscopy (CLSM) imaging further confirmed a significant boost in proton transport both near the substrate and across the LS films—unlike Nafion films. LS 1.7 achieved molecular orientation less parallel to the substrate due to its non-linear branched architecture, facilitating efficient ion conduction in both in-plane and out-of-plane directions. In contrast, excessive cross-linking in LS 2.9 (as suggested by contact resonance AFM (CR-AFM)), and poorly developed ionic domains in LS 0.6 films likely hindered their thin -film proton conductivity. This work identifies key factors beyond IEC, including domain connectivity, swelling behavior, molecular geometry and alignment, that influence thin-film proton transport. Overall, LS ionomers present a compelling, bio-based alternative to Nafion, advancing both performance and sustainability in proton exchange membrane fuel cells.
